This invention relates to the fabrication of apertured insulation films, and more particularly to the fabrication of integrated circuits having such films as insulation between metallization levels.
The advent of very large scale integrated circuit technology places increasing demands on multilevel interconnection systems. As the minimum features become smaller, e.g., two micron gates in 64K random access memories, it has become necessary to form more than one level of metallic interconnections requiring adequate electrical insulation or isolation between the various levels of contacts at crossover points and ohmic connection between the various levels through apertures in the insulating layers. The feed-through holes, or vias, in the insulating layer between metal levels are required to have tapered sidewalls in order to minimize step-coverage problems and thus enhance yield and reliability.
One method for producing tapered apertures or vias in silicon dioxide is known as the "resist erosion" process wherein a photoresist is formed on an oxide layer. This process is described in the following articles: J. A. Bondur and H. A. Clark, "Plasma Etching for SiO.sub.2 Profile Control", Solid State Technology, April 1980, pp. 122-128; N. S. Viswanathan, "Simulation of Plasma-Etched Lithographic Structures", Journal of Vacuum Science Technology, vol. 16, No. 2, March-April 1979, pp. 388-390. Tapered apertures are formed in the resist extending to the oxide which is then exposed to a nonselective etchant. As the resist and the oxide are etched the opening in the resist widens which produces an aperture in the oxide layer having tapered sidewalls. This is essentially a pattern transfer process since the initial taper in the resist is reproduced in the oxide layer, assuming a one to one resist-oxide etch ratio. A disadvantage of this method is that the thickness of the resist and the initial taper of the aperture therein must be precisely controlled in order to produce an aperture in the oxide having the required taper. Further, the repeatability of this process is dependent upon the extent to which the composition of the resist can be controlled from batch to batch. It is also difficult to obtain tapered openings less than about two microns because a correspondingly thinner layer of resist is required which may be insufficient to protect the underlying layer.